1. Field of the Invention
The present invention generally relates to a communication system and a communication apparatus and, more particularly, to a communication system in which a station-side terminal apparatus and multiple subscriber-side terminal apparatuses are connected via a coupler.
2. Description of the Related Art
An example of a communication system in which a station-side terminal apparatus and multiple subscriber-side terminal apparatuses are connected via a coupler includes a LAN system built with optical paths having multicast function, for example. FIG. 1 shows such a LAN system including a station-side optical terminal apparatus (L2-SW for example) 10 and a multicast router 50. “L2-SW” means a known layer 2 LAN switch provided with multiple subscriber ports 1, 2, . . . , n. A host apparatus (a personal computer PC for example) is connected to each subscriber port via a subscriber optical terminal apparatus 21, 22, . . . , 2n. 
When distributing multicast packet data, the multicast router 50 generally determines whether there is a host apparatus that desires to receive the multicast packet data is connected to a port, and distribute the multicast packet data to the port to which a host apparatus that desires to receive the multicast packet is connected. A packet called Host Membership Query (hereinafter referred to as HMQ) is used for determining whether there is a host apparatus that desires to receive the multicast packet data is connected to a port, and a packet called Host Membership Report (hereinafter referred to as HMR) is used for responding to the HMQ. This HMQ is periodically transmitted from the multicast router regardless of whether distribution is made or not in order to establish procedures for new distribution of multicast data and inquire a host apparatus whether to continue to receive distribution. The above procedures are referred to as Internet Group Management Protocol (IGMP). For example, patent document No. 1 discloses a communication system using the IGMP.
Since the IGMP is a upper rank protocol to the function of L2-SW 10, L2-SW 10 does not analyze IGMP packets. As a result, if there is a host apparatus Hi accommodated to the L2-SW 10 that is receiving multicast distribution, processing of the multicast distribution is added to communication processing of the L2-SW 10. So-called IGMP snooping technique is applicable to this problem. The IGMP snooping allows the L2-SW 10 to acquire and analyze IGMP packets which are essentially in the upper layer than the L2-SW 10, and distribute data to particular ports of the L2-SW based on the result of the analysis. If such IGMP snooping technique is used, the L2-SW 10 can identify the particular ports of the L2-SW 10 to which the data is to be distributed by snooping the HMR from the host apparatus Hi, and selectively transfer the multicast packet data only to the particular ports.
FIG. 2 is a schematic diagram showing a so-called Ether PON system as an example of the LAN system using optical transmission paths in which multicast function is employed as described above. The Ether PON system is a communication system in which known Ethernet (trade mark) technology and PON (Passive Optical Network) technology are integrated. In the case of the Ether PON system, subscriber-side ports 1, 2, . . . , n−1, n are connected to multiple subscriber-side terminal apparatuses (referred to as Branch Units, or BU in short).                (Patent Document 1) Japanese Patent Laid-Open Application No. 2000-4251        
As shown in FIG. 1, in the case of a conventional L2-SW that does not use optical couplers, since there is only one host apparatus accommodated to one port, data can be distributed using the IGMP snooping without any problem. However, in the case of the PON system shown in FIG. 2, there are multiple host apparatuses accommodated to a port via an optical coupler. As a result, even if a particular port is identified by the snooping analysis of HMR, the L2-SW 10 cannot determine which host apparatus accommodated to the particular port has transmitted the HMR. Consequently, the multicast packet is transmitted to all host apparatuses H1, H2, . . . , Hk accommodated to the particular port 1, for example, as shown in FIG. 2. This multicast packet reaches to the layer 3 terminal function unit of all host apparatuses, but only the host apparatus that has transmitted the HMR (the host apparatus Hi in this case) receives the multicast packet (distribution data) at application level. However, the other host apparatuses that have not transmitted the HMR (the host apparatuses H1, . . . , Hi−1, Hi+1, . . . , Hk in this case) also need to handle the distribution data unnecessarily.
FIG. 3 is a schematic diagram showing the flow of the HMR in the communication system shown in FIG. 2. Even if there are multiple host apparatuses Hi through Hj requesting for the distribution data accommodated to the same router 50, IGMP requires that only one of the HMRs transmitted by the multiple host apparatuses be snooped. Therefore, the communication system is designed such that only a representative host apparatus transmits a HMR and the other host apparatuses do not need to transmit HMRs even if the other host apparatuses desire to receive the distribution data. That is, according to the specification of IGMP, it is required that each host apparatus transmits a HMR a random time period measured by its random timer after the receipt of a HMQ from the router. This requirement allows a host apparatus the random timer of which times-out at first transmits a HMR as a representative to all ports of the L2-SW 10 and the router 50. The other host apparatuses that is to transmit HMRs after the time out of their random timer cancel their transmission of the HMRs in response to the receipt of the HMR transmitted by the representative host apparatus through the ports of the L2-SW 10.
Thus, once a HMR is transmitted from a representative port, the transmission of HMRs from the other ports is canceled. The transmission of a HMR from a host apparatus that desires to receive distribution data is also canceled. As a result, the transmission of distribution data to the host apparatus that desired to receive distribution data may be terminated.
If a host apparatus requests to terminate data distribution, the host apparatus transmits a Leave packet indicating the request to terminate data distribution. In response to receipt of the Leave packet, the router 50 performs a sequence for confirming the termination of data distribution. The router 50 transmits a HMQ for confirmation at first. If the L2-SW 10 receives HMRs less than a predetermined times from a particular port in response to transmission of the HMQ, the L2-SW 10 terminates the data transmission to the particular port. In this case, the request to terminate data distribution from a host apparatus may be disregarded, and consequently the data distribution may be continued. The host apparatus that desires to terminate the data distribution needs to receive the distribution data unnecessarily. That is, after the host apparatus transmits the Leave packet, the L2-SW 10 needs to perform the sequence for confirmation as described above. Accordingly, the L2-SW requires a time period for performing the sequence for confirmation before terminating the data distribution.